Re: [Boost-users] Shared pointer over-write

From: Elli Barasch (comptonsw_at_[hidden])
Date: 2007-12-19 12:12:00

That's the answer I was looking for.  Sorry for my mangled example, but 
you had the idea...

Nat Goodspeed wrote:
> Elli Barasch wrote:
>
>   
>> Shared pointer question:
>>     
> [snip odd example]
>
>   
>> Will this cause a memory leak?  Do I need to do a delete in between, or 
>> does the act of reassignment cause the prior object to be dereferenced?
>>     
>
> A second assignment to a given shared_ptr should not cause a leak. You 
> should avoid an explicit delete.
>
> If p is a shared_ptr pointing to a heap object, then assigning a new 
> value to p will decrement the previously-referenced object's refcount, 
> which may cause the heap object to be deleted.
>
> This document:
> http://boost.org/libs/smart_ptr/shared_ptr.htm#Members
> says this under 'assignment':
>
> shared_ptr & operator=(shared_ptr const & r); // never throws
> template<class Y> shared_ptr & operator=(shared_ptr<Y> const & r); // 
> never throws
> template<class Y> shared_ptr & operator=(std::auto_ptr<Y> & r);
>
>      Effects: Equivalent to shared_ptr(r).swap(*this).
>
>      Returns: *this.
>
>      Notes: The use count updates caused by the temporary object 
> construction and destruction are not considered observable side effects, 
> and the implementation is free to meet the effects (and the implied 
> guarantees) via different means, without creating a temporary.
>
> But the important point is that the assignment must behave as though a 
> shared_ptr holding p's prior value has been deleted.
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